The present invention relates to a control system for an electromagnetic clutch for a motor vehicle.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A No. 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes for clutch torque (clutch current). One of the modes is selected in accordance with a position of a selector lever and driving conditions of the vehicle to control the electromagnetic clutch.
At a start of the vehicle, the clutch is controlled by the clutch torque raised in proportion to the engine speed. Slipping of the clutch at the start of the vehicle depends on the engine torque and the clutch torque. FIG. 7 shows a clutch torque curve Tc and engine torque curve Te at an opening degree of a throttle valve. The engine torque coincides with the clutch torque at a point A (stall speed Ns). Thereafter, until input speed of the clutch coincides with output speed thereof and the clutch is entirely engaged, engine speed is kept constant. However, the engine stall speed is influenced by variances in clutch torque characteristic, which are caused by manufacturing tolerances and deterioration with time, or the reduction of engine torque at high altitudes.
If the engine torque reduces, the engine stall speed Ns decreases to a lower point as A'. The vehicle starts at a small engine torque range so that the driveability decreases. If the clutch torque reduces, the engine stall speed Ns increases to a point A", which causes reduction of transmission efficiency and a rise of the temperature of the clutch.
In order to solve these problems, a control system is provided for the engagement of clutch, for example, disclosed in Japanese Patent Application Laid Open No. 60-37425. In the prior art, a desired engine stall speed is determined in accordance with the load on the engine at a start of the vehicle, and the clutch torque is controlled to coincide engine speed with the desired engine stall speed.
As shown in FIG. 8, at a start of a vehicle, the engine speed Ne increases to a point A of the desired engine speed in accordance with the engine load, remaining constant thereafter. A clutch output shaft speed Nc increases in accordance with running resistance of the vehicle or gear ratios of the transmission. When both speeds coincide with each other at a point B, the clutch is engaged. Thereafter, both speeds Ne and Nc increase together.
In such a characteristic, the increase of engine speed is rapidly stopped at the point A. Further, the engine speed is changed discontinuously at the point B. These actions cause shocks on the vehicle. Since the clutch is uniformly controlled by a predetermined engine speed, it is difficult to provide a sufficient acceleration efficiency.